The present invention claims the benefit of the filing date of Japanese Patent Application Serial No. H10-357583, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
This invention relates to a method for forming a helical antenna having an antenna element which is covered with, for example, an insulating layer, and more particularly, to a method for forming a helical antenna, in which the antenna element is integrated into the insulating layer by monolithically moulding the insulating layer around the antenna.
2. Description of the Related Art
In recent years, helical antennas having helical antenna elements (or aerial elements) are widely used in portable communication devices, such as cellular phones, because of their wide-band characteristics and the advantage of requiring less space.
An antenna element is generally attached to a portable communication device so that it projects from the case. Accordingly, the antenna element is covered with an insulating layer for purposes of preventing the antenna element from deforming due to external forces, and preventing the resonant frequency changing.
FIGS. 9 and 10 illustrate a conventional method for forming a helical antenna having an antenna element which is covered with a monolithically moulded insulating layer.
The conventional antenna element 101 is formed by helically coiling a conductive material. One end of the antenna element 101 is fixed to a metal fitting 102. The antenna element 101 is electrically connected to the aerial coupled circuits and the transmitting/receiving circuit of a portable communication device (not shown) via the metal fitting 102 and the feeder 103 connected to the metal fitting 102.
The coil pitch P of the antenna element 101 is determined based on the resonant frequency of the helical antenna. The coil pitch is almost the same over the entire length of the antenna along the longitudinal axis.
In order to cover the antenna element 101 with the monolithically moulded insulating layer 104, a core pin 105, which functions as a moulding die, is inserted from the free end of the antenna element along its longitudinal axis, as shown in FIG. 9.
The antenna element 101, in which the core pin 105 was inserted, is accommodated in the cavity 107 which is defined by the top and bottom dies 106a and 106b, as illustrated in FIG. 10. The antenna element 101 is supported by the core pin 105 in the cavity 107 so that a prescribed gap is generated between the inner surface of the cavity 107 and the antenna element 101.
Then, a molten resin, which is the material of the insulating layer 104, is injected into the cavity 107 from the gate G to fill the gap between the antenna element 101 and the moulding dies 106a and 106b. 
When the moulding resin is cured, the antenna element 101 is integrated into the insulating cover 104, whereby a helical antenna with an anternal element covered with the insulating layer is completed.
However, the conventional method has a problem that the injection pressure of the moulding resin, which forms the insulating layer, causes the coil pitch P of the antenna element 101 to change, and consequently, the electric parameters of the antenna deviate from the designed values.
For example, in FIG. 9, the pitches P1, P2, and P3 measured at three different positions (i.e., near the base, in the middle, and near the tip) along the coil axis before the moulding of the insulating cover are 2.49 mm, 2.43 mm, and 2.464 mm, which are close to each other with the offsets within the acceptable error. However, with the injection gate located near the metal fitting 102, as illustrated in FIG. 10, the pitches P1xe2x80x2, P2xe2x80x2, and P3xe2x80x2 of the antenna element 101 after the moulding become 3.18 mm, 2.68 mm, and 1.44 mm, and the end portion of the antenna element 101 is undesirably compressed. This pitch divergence inevitably occurs even if the gate position is adjusted. Thus, the resonant frequency of the helical antenna greatly deviates from the designed value depending on the moulding conditions, which results in a low product yield in mass production.
In order to overcome this problem, Japanese Patent Application Laid-open No. 8-894017 discloses a helical antenna, which can prevent the pitch divergence of the antenna element 111 by forming a helical guide groove 113 in the insulating cap 112 in accordance with the coil pitch P of the antenna element 111, as shown in FIG. 11, and which can protect the antenna element 111 from external forces by bonding an insulating cover 114, which was formed in advance, around the antenna element 111 by adhesive, or by fitting the antenna element 111 into the insulating cover 114, as shown in FIG. 12.
However, the helical antenna 110 disclosed in 8-894017 requires an assembling step or a bonding step for attaching the insulating cover to the helical antenna. In addition, the bonded or fitted insulating cover cannot sufficiently protect the anternal element. If the communication device is dropped, the insulating cover 114 is likely to break or to be disengaged from the antenna element 111, and as a result, the antenna element is exposed.
The present invention was conceived in order to overcome these problems in the prior art, and it is an object of the invention to provide a method for forming a helical antenna with an antenna element covered with a monolithically formed insulating layer, which can reliably protect the antenna element from external forces, and can maintain the electric properties of the antenna element constant.
It is another object of the invention to provide a method for forming a helical antenna which does not require an extra assembling step for attaching an insulating cover to the antenna element, and which can prevent breakage or disengagement of the insulating cover, as well as undesirable exposure of the antenna element.
In order to achieve these objects, in one aspect of the invention, a helical antenna having a helical antenna element covered with an insulating layer is formed so that the antenna element and the insulating layer are monolithically moulded, and that the base of the antenna element is supported by a metal fitting. This method comprises the following steps:
(a) inserting the antenna element into an cylindrical cavity of an first moulding die, the inner diameter of the cylindrical cavity being equal to or slightly greater than the outer diameter of the antenna element;
(b) injecting an first insulating resin into the cylindrical cavity to produce a primary moulded product in which the antenna element is monolithically integrated into the first insulating resin;
(c) putting the primary moulded product in a cavity of a second moulding die so that a prescribed gap is formed between the outer surface of the primary moulded product and the inner surface of the second moulding die; and
(d) injecting a second insulating resin into the cavity of the second moulding die so that the cylindrical surface of the primary moulded product is monolithically covered with an insulating layer made of the second insulting resin, whereby the antenna element is completely covered with the monolithically formed insulating layer.
Because the inner diameter of the cylindrical cavity is equal to or slightly greater than the outer diameter of the antenna element, the antenna element is inserted into the cavity with its outer surface is in contact with the inner surface of the cylindrical cavity. When the first insulating resin is injected in the cylindrical cavity, the injection pressure causes the antenna element to expand outward in the radial direction, which further causes a friction force between the antenna element and the inner face of the moulding die. This friction force prevents the antenna element from moving along its longitudinal axis (or the coil axis). When the first insulating resin is set, the antenna element having a constant pitch is completed as the primary moulded product, with little deviation.
This primary product is retained in the second moulding die, and the second moulding resin is injected in the gap between the second moulding die and the primary product in order to monolithically form the insulating layer over the outer surface of the primary product. Thus, the antenna element is covered with the insulating layer in such a manner that the antenna element and the insulating layer are integrated into a single body of the helical antenna.
Since the pitch of the antenna element is fixed by the cured first moulding resin, it does not change even if the injection pressure of the second moulding resin is applied to the primary product. Accordingly, the final product, that is, the helical antenna can have the designed electric properties. In addition, the antenna element is completely covered with the insulating layer in the monolithic manner, it is durable against an impact, and undesirable exposure of the antenna element can be prevented.
In another aspect of the invention, a helical antenna having a helical antenna element covered with an insulating layer is formed by the following steps:
(a) inserting a core pin of a moulding die into a cylindrical shell made of an insulating resin;
(b) inserting the cylindrical shell which received the core pin of the moulding die in the center hole, into the antenna element along the longitudinal axis of the antenna element;
(c) retaining the antenna element in a cavity defined by the moulding die so that a prescribed gap is formed between the antenna element and the inner surface of the moulding die;
(d) injecting a molten resin into the cavity to form an insulating cover which monolithically integrates the antenna element and the cylindrical shell;
(e) removing the core pin from the cylindrical shell, and obtaining a cylindrical antenna;
(f) covering the aperture of the cylindrical antenna with an insulating cap, whereby the helical antenna having the antenna element completely surrounded by the insulating cover is completed. In this case, the antenna element and the insulating layer are again monolithically moulded, and the base of the antenna element is supported by a metal fitting.
When the molten resin is injected into the cavity, it flows along the outer surface of the cylindrical shell inserted in the antenna element. Upon contacting with the molten resin, the cylindrical shell thermally expands and retains the inner face of the antenna element, whereby the pitch of the antenna element can be kept constant even if the injection pressure of the molten resin is applied to the antenna element.
When the moulding resin which filled the gap between the inner wall of the moulding die and the antenna element is cures, the core pin is pulled out. The resultant cylindrical antenna comprises the cylindrical shell, the antenna element, and the insulating cover which are all integrated into a single unit.
The opening of the cylindrical antenna is covered with an insulting cap. Since the pitch of the antenna element is kept constant, the designed electrical properties can be achieved with little deviation. This monolithically formed helical antenna is durable against external forces or impacts, and can prevent the antenna element from being exposed.
Preferably, the insulating resin forming the insulating cover is heated and molten at a temperature higher than at least the melting point of the insulating resin of the cylindrical shell, and the molten resin is injected into the cavity.
Because the temperature of the molten resin injected into the cavity is higher than the melting point of the cylindrical shell made of another type of insulating resin, the cylindrical shell gets softened upon contacting with the injected resin, and the helically coiled antenna element digs into the softened shell surface. Thus, the antenna element is retained by the cylindrical shell with its initial pitch kept constant.
When the moulding resin is set, the core pin is removed. Then, a helical antenna, in which the insulating cover, the antenna element, and the cylindrical shell which was softened and cured again are monolithically integrated into a single unit is completed.
Preferably, an insulating cap is monolithically formed at the opening of the cylindrical antenna by retaining the cylindrical antenna, from which the core pin has been removed, in a second moulding cavity, and by injecting a second moulding resin to form the insulating cap.
Because the insulating cap is integrated into the cylindrical antenna by injection-moulding, it does not come off even if external forces are applied. In addition, no additional steps for bonding or fixing the cap to the cylindrical antenna.